The present invention relates to X-ray tubes. In particular, the present invention relates to a method and apparatus for reducing high voltage breakdown events in X-ray tubes.
X-ray imaging systems have long been available to doctors as a valuable tool for examination and diagnosis. X-ray imaging systems rely on an evacuated high voltage (e.g., 30-150 kV) X-ray tube. The X-ray tube produces an X-ray beam by generating an electron beam at the tube cathode, focusing the electron beam through a focus grid, and impacting the electron beam upon a tube anode. A steady, predictable X-ray beam greatly enhances the diagnostic usefulness of an X-ray system. However, past X-ray tubes suffered from a deleterious effect called high voltage breakdown or vacuum arcing that interrupted the steady X-ray beam.
The prevailing theory on electrical breakdown of the vacuum gap in the X-ray tube is predicated on the intensification of the electric field near the cathode surface caused by positive ion space charge formation in the region above the cathode surface. The electric field intensification results in an increase in localized currents from field emission sites on much of the cathode surface as well as neutralization of negative thermionic space charge about the filament that serves to reduce the electrostatic shielding of emitters found in that region. When the current density from an emitter is high enough to cause substantial Joule heating of the emitter tip, the constituent emitter material can sublime into the vacuum gap where it can be ionized. Ensuing plasma formation and high voltage breakdown results in the vacuum gap across the gap between the cathode to anode.
High voltage breakdown events short circuit the X-ray tube and interrupt the X-ray beam. In order to mitigate the interruptions, X-ray tubes undergo an extensive burn-in procedure after manufacture. The burn-in procedure attempts to eliminate, through electrical discharge, cathode field emission sites by allowing high voltage breakdowns to occur in a controlled fashion. While the burn-in procedure helps to reduce high voltage breakdowns in installed X-ray systems to a certain extent, the burn-in procedure does not completely eliminate all field emission sites. As a result, installed X-ray systems continue to experience high voltage breakdowns and the resultant interruptions in the X-ray beam.
A need has long existed in the industry for a method and apparatus for reducing high voltage breakdown events in X-ray tubes that addresses the problems noted above, and others previously experienced.
A preferred embodiment of the present invention provides an X-ray tube subsystem including an X-ray tube and a grid voltage supply. The X-ray tube provides a grid bias connection, a filament bias connection, and an anode bias connection. The grid voltage supply is connected to the grid bias connection and filament bias connection, and is adapted to produce an ion collection voltage substantially less than an electron beam focus voltage, to sweep free ions out of the X-ray tube.
Another preferred embodiment of the present invention provides a method for operating an X-ray system to reduce high voltage breakdown events. The method includes the steps of providing an X-ray tube that includes a grid bias connection and filament bias connection. In addition, during X-ray tube operation, the method creates an ion collection voltage between the grid bias connection and the filament bias connection that is substantially less than an electron beam focus voltage, to sweep free ions out of the X-ray tube.